Tuning fork with its frequency immune to the effects of gravitation and acceleration



y 1959 F. DOSTAL 2,894,188

TUNING FORK WITH ITS FREQUENCY IMMUNE TO THE EFFECTS OF GRAVITATION AND ACCELERATION Filed Feb. 9, 1956 FIG. 3

'//vv- TOR FRANK DOSTAL J; r TORNEY United States Patent Frank Dostal, Great Neck, N.Y., assignor to American Time Products, Inc., New York, N.Y., a corporation of Delaware Application February 9, 1956, Serial No. 564,497

7 Claims. (Cl. 318-127) The instant invention relates to providing a tuning fork with a precise and unvaried frequency at all times independent of the attitude or position of the fork and of the accelerating motion to which it may be subjected. For many control purposes, independency of the frequency of an oscillator in the 200 to 1000 cycle range is most desirable and essential. Even an oscillator of such accurate frequency generation as the oscillators according to US. Patents 2,707,234, entitled Tuning Fork Oscillators, and issued to the instant applicant on April 26, 1955, requires additional expedients to overcome these types of errors to which all tuning fork oscillators are subject.

It is an object of the instant invention to provide a supporting means for an electrical tuning fork oscillator by means of which such errors are eliminated, and the control frequency is thus at all times, irrespective of the direction and magnitude, independent of the acceleration of the structure in which the tuning fork is mounted and which the fork frequency controls.

It is well known that all tuning forks are subject to errors due to gravity because of the pendulum effect. Thus, for a given 400-cycle fork with the axis of its tines vertical and the tines facing upwardly, there is an error of -l7 parts per million compared to the fork rate with its'tines horizontal, and the oscillator is slow by less than 2. seconds in twenty-four hours. When the fork is positioned' with its tines axis vertical and the tines downwardly, this error is +17 parts per million, that is, there is a total change of 34 p.p.m. for a total change of twice gravity g. This may be expressed by Dg=K(l-cos where D; is the deviation rate due to gravity g in parts per million, K is a contant depending on the fork frequency, and qt is the angle the fork axis makes with the vertical up position. Thus for a fork with its axis vertically up, =0 and D =0; for the fork axis horizontal =90 and D =K; and for the fork axis vertically down =18Q and D =2K.

With a tuning fork moving along the tine axis at an acceleration measured. in terms of g, the vertical acceleration due to gravity, the error in the rate is multipled by the acceleration factor, that is D =KG cos 6 (2) where D is the deviation due to the accelerating force applied in parts per million, G is the number of times the linear acceleration is greater than the force of gravity g, and 0 is the angle the fork axis makes with the direction of the applied force. This Equation 2 is readily recognizable as the more general 01m of Equation 1 above, in whichi-gravityacting vertically is the special case. Thus the 400-cycle fork above mentioned, when 2,894,188 Patented July 7, 1959 moving along its tine axis under an acceleration of 10 g. along such axis will have an error of p.p.m. An error of such magnitude would, of course, be intolerable in many applications, for example in military missiles and aircraft, for fire control, guidance and navigation.

Considering the generalized Equation 2, the error due to linear acceleration along the tine axis can be caused to disappear. Where the accelerating force acts constantly in the same direction, the fork can be mounted fixed in the moving device or vehicle with its tine axis perpendicular to the direction of such motion. So doing reduces D to zero, and only D, would remain if free to act. But it is likewise true that D the special case, can be caused to disappear by mounting the tine axis of the fork vertical to the direction of gravity. But the acceleration due to gravity is always vertical, and hence the fixed plane in which the tine axis must be mounted to have its effect on D, disappear is horizontal. It is, however, quite obvious that the accelerated movement of devices in which precise frequency of the tuning fork oscillator is required is rarely purely vertical such as would make feasible a fixed mounting of the fork with its time axis in the horizontal plane. The accelerated motion is nearly always in other directions, as also in varying directions, so that a fixedly mounted fork would produce errors in most instances.

It is obvious that generally in practical applications, the fork is exposed to the effect of gravity and of an accelerating force in some direction and magnitude other than those of gravity, which force has a vertical and a horizontal component. The force due to gravity is added to, or subtracted from, the vertical component of the accelerating force, and, with the horizontal component of the accelerating force, produces a resultant R of which the direction and magnitude differ, in the general case, from those of both gravity and the applied accelerating force. Thus, it becomes a problem of supporting. the tine axis and fork so that such axis is at all times perpendicular to the resultant R at any given moment and at each moment, in order to eliminate, as above shown, the errors due at each moment to both gravity and the applied accelerating force.

I am so able to maintain the fork in this attitude, that is, at all times perpendicular to the resultant R, by mounting the fork in a girn'bal support free to move in the three orthogonal axes and with the center of gravity of the fork mounting located symmetrically below the geometric center of the three axes. I provide electrical commutation means on each of the ginrbal trunnions to enable connection of the fork driving coil and the fork output means to its associated electrical equipment. where a transistorized fork is used, the relatively small battery power source required therefor may readily be positioned in the weighting extension of the fork housingv housing assembly with space therebetween which is filled.

with a liquid, such as various of thesilicone oils.

The foregoing, and other, objects and features will readily be understood from the following description of an illustrative embodiment of my instant invention, taken.

together with the appended drawing, in which Figure l is a plan view, highly schematic, of gimbals mounted, electrically driven, tuning fork oscillator,

in accordance with my present invention;

Figure 2 is a section along line 22 of Figure 1;

In the case Figure 3 is a vector diagram of the forces acting on the fork in spatial movement; and

Figure 4 is a section similar to that of Figure 2 showing the position of the fork and weight when the accelcrating force of Figure 3 acts on the body in which the instant oscillator is mounted.

The tuning fork is an assembly of which the detailed elements which are not here shown are preferably assembled in a hermetically sealed enclosure 11, corresponding to those shown in above identified US. Patent 2,707,234 and herewith incorporated by reference, which enclosure in turn is supported spaced from the walls of the sealed housing 12. It is to be noted that the tine axis of fork 10 is parallel to the axis of the housing 12. Perpendicular to the axis of the housing 12 and from its midlength region a pair of diametrically opposite trunnions 13 and 14 extend horizontally and diametrically opposite each other therefrom and are journalled for ready rotation in the ring 15. The axis of the tuning fork 10, as vw'll be noted from Figure 1, is coincident with a diameter of the ring 15 perpendicular to the alignment of the trunnions 13 and 14. Ring 15 is of such diameter and height that the sealed housing 12 may readily make a complete revolution on its trunnions 13, 14 without the housing 12 or its centrally extending weighted projection 16 striking the ring. Ring 15 in turn has a pair of diametrically opposite outwardly extending trunnions 17 and 18 which are along a diameter of the ring 15 parallel to the axis of the tuning fork, and hence at 90 degrees from the trunnions 13 and 14. The outer end of each trunnion 17 and 18 is in turn journalled for free rotation in the ring 19, which in turn has trunnions 20 and 21 directly opposite each other and at 90 degrees from the trunnions 17 and 18. Trunnions 2t! and 21 have their respective outer ends journalled for free rotation in the frame 22 which is resiliently suspended within the device housing 23 by a plurality of shock mountings 24, as indicated in the upper right portion of Figure 1. It will be noted that all the trunnions pairs, 13 and 14, 17 and 18, and 20 and 21, are substantially coplanar horizontally and with the tuning fork 10, and that the trunnions 13, 14, 20 and 21 are aligned. Projection 16 from the fork assembly enclosing housing 12 is centrally disposed at the midlength of the enclosure, and is vertically aligned and perpendicular to the trunnions 13 and 14. It is weighted sufficiently that it will always position itself below the tuning fork housing, as shown in Figure 2, and, as has above been stated, may together with the housing be filled with selected silicone liquids having substantially constant viscosity over a large temperature range.

A pair of electrical terminals 25 is positioned on the frame 22 and by way of a pair of brushes connect to a pair of mutually isolated slip rings 26 on the trunnion 20. The pair of slip rings 26 in turn is electrically connected to the pair of slip rings 27 on trunnion 17, slip ring pair 27 in turn being electrically connected to brushes riding on slip ring pair 28 on trunnion 13, and the latter pair 28 being connected, for example, to the driving coil 29 of the tuning fork. Similarly a pair of terminals 30 on frame 22 is electrically connected to the slip rings 31 on trunnion 21, which slip ring pair 31 in turn is connected by appropriate conductors and brushes to slip ring pair 32 on trunnion 18, which ring pair 32 in turn is similarly connected to the slip ring pair 33 on trunnion 14, which in turn is connected, for example, to the pick-up coil 34 of the tuning fork. Obviously the various conductors from each terminal pair 25 and 30, the intervening brushes and slip rings, and the coil terminals are appropriately insulated to provide separate circuits which are continuous.

In the operation of the device incorporating the tuning fork and mounting as above described by way of illustrative embodiment, assume that, as shown in the vector diagram of Figure 3, the applied accelerating force is represented in both magnitude and direction by the vector 35. Since the accelerating force shown is such as to move the device upwardly, gravity 36, acting downwardly on the fork positioned at the origin of the coordinates, will be in the direction opposite to the vertical component 37 of the acceleration 35 and hence, as is well known, the resultant of the horizontal component 38 of the applied accelerating force 35 and the vertical forces, 36 and 37, which are subtractive, will be the resultant 39. The fork 10 thus as the result of its gimbals mounting and also its weighted projection 16 takes on the position perpendicular to the resultant force 3? at all times. This is shown by way of example in Figure 4 for the accelerating force of Figure 3, where the tuning fork housing 12 has moved to the position perpendicular to the resultant 39. When the general direction of the accelerated movement is downward, the same is true, with the vertical component of the applied accelerating force and the force of gravity now being additive. Thus at all times and at any given time during accelerated spatial movement of the device in which the tuning fork 10 is so mounted, there will be no error whatsoever in the rated frequency of the tuning fork oscillator.

While I have illustrated my invention by a particular embodiment thereof, my invention is not limited in its application to the specific apparatus and particular arrangement herein disclosed. The terms and expressions which I have employed in reference to my invention are used as terms of description and not of limitation, and I have no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or any parts thereof, but on the contrary, intend to include herein any and all equivalents, modifications and adaptations which may be employed without departing from the spirit of the invention.

What I claim is:

1. A tuning fork oscillator comprising a tuning fork, a fork driving coil, a pick-up coil, an enclosure within which the fork and both coils are integrally supported, a first ring, a first means pivotally supporting the enclosure within the first ring and permitting unobstructed rotation of the enclosure thereabout, the tuning fork being coplanar with the first pivot means, a mass weighting the enclosure to position its center of gravity below the first pivot means, a second ring, a second means pivotally supporting the first ring within the second ring and coplanar with and perpendicular to the first pivot means and permitting unobstructed rotation of the first ring within the second ring thereabout, a frame, a third means pivotally supporting the second ring within the frame and coplanar with and perpendicular to the second pivot means and permitting unimpeded rotation of the second ring within the frame thereabout, and individual connections, electrically isolated from but supported by the rings, from the frame to the driving and pick-up coils.

2. A tuning fork oscillator according to claim 1 in which each of the pivotal support means is a pair of trunnions integral with the supported element and rotatably journalled in the supporting element.

3. A tuning fork oscillator according to claim 1 in which each of the pivotal support means is a pair of trunnions integral with and transversely aligned on the supported element and rotatably pivoted in the supporting element, and the individual electrical connections for each ccil are by way of a portion of each ring and a pair of brushes engaging one of a pair of spaced slip rings on one trunnion of each pair of trunnions, the slip rings of a pair being insulated from each other.

4. A tuning fork oscillator according to claim 2 in which the frame is resiliently supported on the walls of a housing.

5. A tuning fork oscillator according to claim 1 in which the mass weighting the enclosure is in the form of an extension integral with the central bottom region of the enclosure and aligned perpendicularly with the first pivotal means.

6. A tuning fork oscillator according to claim 1 in which a sealed casing is fixedly positioned within and spaced from the walls of the enclosure, the casing housing the fork and at least the driving and pick-up coils, and the region between the casing and the enclosure is filled with a liquid having substantially constant viscosity over a wide temperature range.

7. The method of eliminating deviation in the frequency of a tuning fork oscillator during the spatial move- 6 ment of the fork as a unit under an applied accelerating force, comprising providing the tuning fork with an en closure and weighting the enclosure so that center of gravity of the assembly lies in a plane below the fork, and suspending the enclosure assembly in gimbals to position said center of gravity below the plane of suspension.

No references cited. 

